It’s funny how an established player like Microsoft can be described as disruptive, as that term is usually used to describe pal Michael Robertson who first established MP3.com and Lindows, before starting GizmoProject.
While it may be semantics, I think what Microsoft is doing in VoIP is really an evolutionary move and logical extension of their eco-system based upon Exchange, with the only real difference being they have integrated voice exhange for the enterprise and plan to use their installed user base of Exchange servers to help propel the new voice offerings.
Companies in the current PBX and Hosted IP-PBX world do need to be wary, but then again, with the Sprint WiMax news earlier this week, the OS of choice their will likely win the day. I expect that to be Linux.